Chan e dealbh dona a th’ ann
You cannot use tha, a bheil, and chan eil (forms of the verb bi) to say what or who someone or something is, you can only use them do describe things and people (like in it is bad, he is nice; but not in it is a bad thing, he is a nice person).
For saying who or what sb or sth is, you need the copula is (which in negative is just cha(n)), eg. is cù e it is a dog, but nowadays it’s common to use a more sophisticated phrase for such sentences, is e cù a th’ ann it is a dog, lit. a dog is it-that is in-it, a dog is in it/him; or chan e cù a th’ ann it is not a dog (a dog is not what is in it).